It is known, with the aim of modifying the lift of the flying surface of an airplane or of modifying its margin vis-à-vis aerodynamic stalling, to equip said flying surface with standard lift-enhancing devices (leading edge slats and/or trailing edge flaps) which are deployable and retractable. These lift-enhancing devices make it possible to substantially increase the lift of the airplane, when they are deployed, and thus to decrease its approach speed.
The pilot of an airplane configures, with the aid of a standard control member, the so-called slats/flaps lever, said lift-enhancing devices into the position of his choice, as a function of the conditions (speed, altitude, etc.) and of the phases of flight (rolling, takeoff, climb, cruising, descent, standby, approach, landing). The positions of the lift-enhancing devices vary progressively between a first position corresponding to complete retraction of said slats and flaps (“cruising” position) and a second position corresponding to complete deployment of said slats and flaps (“landing” position) so as to be able to define several known configurations of the airplane. A given configuration of the airplane therefore corresponds to a particular position of said slats and of said flaps.
These lift-enhancing devices are structurally dimensioned in a known manner, on the basis of the following characteristics:                determination of the minimum flight domain required;        consideration of regulatory wind gusts so as to deduce therefrom the corresponding maximum aerodynamic loads;        application of the other possible loads encountered by the lift-enhancing devices (loads on the ground for example) so as to deduce therefrom the limit loads; and        determination of the associated extreme loads by application of a safety coefficient on the basis of said limit loads.        
Nevertheless, it may happen, in the course of a flight, that the aerodynamic loads applied to these lift-enhancing devices overshoot the limit loads which were used to dimension them, in such a way as to cause significant and irreversible damage (in the form of plastic deformation) to said lift-enhancing devices.
Such situations may be encountered during strong atmospheric disturbances (significant gusts of wind), during unconventional piloting maneuvers (airplane recovery dive) or during erroneous actions on the part of the navigating crew on the member (or lever) for controlling the slats and/or flaps [for example, during the cruising or descent phase, the pilot may move said member for controlling the slats and flaps in error, when he wanted to activate that for the air brakes, the two members being close together]. In this latter situation, a consequence of the pilot's erroneous action would be to produce a considerable nose-up moment on the airplane, difficult for the pilot to counter.
Systems are known which automatically position or move aerodynamic control surfaces of aircraft, such as lift-enhancing devices. By way of illustration, it will be noted that:                document FR-2 425 380 describes a control system which, when an engine of the airplane develops a fault, acts automatically on the control surfaces so as to aerodynamically reconfigure the airplane, in such a way as to compensate for the effect of the loss of thrust on the aerodynamic characteristics of the wing;        document U.S. Pat. No. 4,042,197 describes a device whose aim is to optimize, in the takeoff and approach phase of an airplane, the position of the flaps, as well as the thrust in such a way as to substantially reduce the noise produced by these items of equipment; and        the document FR-2 817 535 discloses a system making it possible to automatically optimize the position of lift-enhancing devices during the takeoff phase of an aircraft so as to reduce the length of runway necessary for takeoff and to reduce the drag, thereby making it possible to obtain a minimum climb slope (with a faulty engine) allowing completely safe takeoff.        
It will be noted moreover that these control systems apply in essence either to the control surfaces or to the trailing edge flaps, and not to the leading edge slats of the airplane. The main reason is that the lift of an airplane is limited by a stalling phenomenon which appears when the angle of incidence of the airplane overshoots a certain angle of incidence value called the “stalling angle of incidence”. Specifically, at high angles of incidence, the flow becomes unstable on the suction face of the flying surface, the air streams detach, resulting in a loss of lift. It is known that the value of this stalling angle of incidence decreases slightly as the trailing edge flaps are deflected. It is for this reason that the leading edge slats are deployed as and when the trailing edge flaps are deployed. However, a system which retracts or deploys said flaps automatically is relatively neutral in terms of margin with respect to stalling and may therefore be regarded as relatively safe vis-à-vis this aerodynamic phenomenon.
On the other hand, the deflection of the leading edge slats is a parameter which strongly influences the value of the stalling angle of incidence. Consequently, going from a deflection Ab to a deflection Cb, with Cb less than Ab, may turn out to be dangerous. Specifically, whereas under the conditions of deflection Ab, the stalling angle of incidence remains far from the flight point, under the conditions of deflection Cb, the airplane may find itself beyond the stalling angle of incidence.
Consequently, since the risk of finding itself in a weak margin situation (or even negative) with respect to the phenomenon of stall is not zero, the position of the leading edge slats is controlled exclusively by manual action of the pilot, via the slats/flaps lever.